Looking at aquatica chart for your camera and lens a diopter is not needed to recreate the focus point under the dome your lens had outside of the housing. Some camera and lens combos under the dome port you would have a farther focus point. But you would get a closer focus point with the diopter if that is what your looking for. So you could get closer to subjects for less backscatter.I have a aquatica d40x housing for my nikon d40x. I use the 16-85mm lens. On the aquatica chart i used a +4 diopter for everything from wide angle to 1:2 macro . For 1:1 macro I add a second +4 diopter for 1:1 macro and smaller. It works very good. Sharp photos. For samples of my work. With the diopter on you can't do half and half shots as the lens would focus underwater but not in air.